Mystery shopping is a $1.5 billion industry that employs more than 1.5 million people worldwide. It has served for decades as a way for companies to measure the quality of the customer service their employees provide.

If you want to find out if your call center is completely free of the long hold times, endless transfers, overly scripted and impersonal conversations, and unempowered employees that usually frustrate customers, you can now hire any one of a growing number of mystery shopping companies to act as your ears on the front lines of customer service.

"The retail industry as a whole is growing, and so are the number of opportunities to interact with customer service reps," says Dan Denston, executive director of the Mystery Shopping Providers Association (MSPA), which has more than 300 member companies worldwide. "As a result, there are a growing number of companies that do telephone performance testing, checking the quality of the customer service rep interactions."

One such company is Shoppers Critique International (SCI), a mystery shopping services provider in Longwood, Fla. Mystery shopping via the phone and electronic channels makes up about two-thirds of the company's business today.

"Most people now go online to research products before a purchase, and then they make contact with the company by email or phone to make specific inquiries," explains Paul Bell, national sales director at SCI.

Mercantile Systems, a Brentwood, Calif.–based mystery shopping services provider, performs about 30,000 telephone shops per year for a range of businesses. It's a growing area for the company, representing about 25 percent of its business today. "We're getting a lot more interest in it," says CEO Dan Cosgrove.

At Confero, a market research and mystery shopping services provider in Cary, N.C., telephone mystery shopping requests have increased in the past few years as well.

The service, according to Janet Morrison, Confero's business development manager, was originally popular in banking, healthcare, and education, but recently "has really been branching out to a lot of different industries because there's more of a focus on improving the customer experience across the board."

The reasons companies need to evaluate their phone support are very basic: For most customers, the phone is still the first point of contact with a company, and there's an expectation that the phone will be answered in three rings or less and that the agent who picks up the call will be warm, enthusiastic, courteous, and capable of addressing callers' issues.

"If the company does not make the best first impression, the customer will go somewhere else," Morrison explains.

Trish Overton, president of Mystery Shoppers, in Knoxville, Tenn., agrees. "In today's economy, you can't afford to lose customers," she says. "How the call is answered could determine whether [customers] make a purchase."

Matthew Kunz, senior director of global brand standards and quality at Sylvan Learning, which operates—either directly or through franchise agreements—about 900 facilities around the country to provide students with tutoring, homework help, study skill building, and test prep, knows that all too well.

"A lot of customers call for more information before coming into one of our centers," he says. "The phone call is incredibly important."

Kunz has used Confero to evaluate his company's telephone reps since late 2011, and last year budgeted about $15,000 for the service. Confero mystery shoppers made about 300 calls to Sylvan's phone reps throughout the year, and Kunz plans to add to that this year. "Some of our franchisees have expressed an interest in having this done monthly," he says.

That's just fine with Kunz, who is a big supporter of mystery shopping. "It's helped us be more responsive," he says. "And our staff likes it because it helps them do their jobs better."

Stacey Paynter, owner of marketing firm Strategic Connections, which is based in Larkspur, Calif., has used mystery shopping to evaluate how agents carry out campaigns. Paynter has been using Mercantile Systems for the past six years.

"I'm a big fan of mystery shopping," she says. "It's an insurance policy for my marketing to let me know if it's working, if my efforts are resonating with customers and converting sales. I can make sure the brand's message is being delivered consistently."

When it's not, Paynter says mystery phone shopping can help quickly uncover operational glitches that need to be corrected. "It's uncovered business opportunities I didn't even know about," she says. "And often, it's what you don't know that can hurt you the most."

Kunz also appreciates the speed at which he can get that kind of information. "I have a six-person staff to do quality control for nine hundred centers, and Confero does things much quicker than I could ever do them internally. It allows me to keep my staff to do what they're paid for. It's a better use of my resources," he says.